The Raspberry Pi is a low cost, credit-card sized computer that plugs into a computer monitor or TV, and uses a standard keyboard and mouse. It is a capable little device that enables people of all ages to explore computing, and to learn how to program in languages like Scratch and Python.
It is capable of doing everything you would expect a desktop computer to do, from browsing the internet and playing high-definition video, to making spreadsheets, word-processing, and playing games.
Actually, Raspberry Pi is not just one computer, but a family. The most recent model is the Raspberry Pi 3 Model B, but other models include the Raspberry Pi Model A, the Raspberry Pi Zero and the Raspberry Pi Zero WiFi.
The Raspberry Pi is essentially a fully fledged server, running Linux. Whilst amazing for the price, this sometimes makes configuring the board difficult for people who may not have a background in Linux. Also, because Linux is a real time operating system, under certain circumstances, the Raspberry Pi can not offer completely accurately timed on/off pulses from its GPIO port. Usually, this is not such a big issue, but there are applications, involving for example, specific types of addressable LED strips where a better choice of controller would be something like an Arduino, which does not have a real time operating system.